Chapter 14: Thermal Physics Flashcards
How is thermal energy transferred?
From hotter regions to colder regions
Thermal Equilibirum
When two substances in physical contact with each other no longer exchange any heat energy and both reach an equal temperature
Conditions for thermal equilibrium
- two regions need to be in contact
- hotter regions would cool down and the cooler region will heat up until same temperature
- final temperature depends on the initial temperature difference between them
Temperature
A number used to indicate the levels of hotness of an object on some scale, require two fixed points of known temperature
Kelvin Scale
Thermodynamic scale, absolute scales that is not defined in terms of a property of any particular substance
Fixed points on the Kelvin scale
- triple point of water - pure ice, water and water vapour all exist at the same temperature and pressure
- absolute zero
How to convert between celcius and kelvin
θ / oC = T / K − 273.15
T / K = θ / oC + 27
Kinetic theory of matter
Model that attempts to explain the properties of the three states of matter
Properties of Solids
held together by strong IMF, closely packed, fixed pattern, vibrate in their fixed position, low energies, very difficult to compress, higher densities
Properties of liquids
weak IMF, closely packed, randomly arranged, can flow past each other, higher energies than particles in solids, do not have a fixed shape, difficult to compress, lower density than solids
Properties of gases
negligible IMF, far apart, randomly arranged, move around in all directions at a variety of speeds, negligible in size, higher energy, not fixed shape or volume, can be compressed, lowest density
Brownian motion
Phenomenon when small particles (such as pollen or smoke particles) suspended in a fluid are observed to move around in a constant random motion
Key points about Brownian motion
• Observed in smoke cells under microscope
• Provides evidence for the existence of molecules in gas or liquids
• A range of speeds between particles
• No preferred directions of movement
What are the limitations of Brownian motion
The observable particles in Brownian motion are slightly bigger than the molecules that cause the motion, air particles cause the observable motion of smoke particles
Air particles are small and light and the smoke particles are large and heavy
Conclusions for Brownian motion
Collisions cause larger particles to change their speeds and direction randomly, effect provides important evidence concerning the behaviour of molecules in gas, especially the concept of pressure
Smaller molecules are able to affect the larger particles as they are travelling at a speed much bigger than the larger particles
They also have a lot of momentum, which they transfer to the larger particles when they collide
What are the two forms of energy
kinetic or electrostatic potential energy
The molecules of all substances contain both kinetic and electrostatic potential energies
Kinetic energy
determined by the speed and mass of the molecules and gives the materials its temperatures
Electrostatic potential energy
due to the separation between the molecules and their position within the structure
What does the amount of KE or EPE depend on?
Amount of kinetic and electrostatic potential energy a substance contains depends on its phase of matter, this is known as internal energy
Internal energy
The sum of the randomly distributed kinetic and potential energies of atoms or molecules within a substance
Factors affecting Internal energy of a system
• Temperature
○ Higher temp means more KE
• Random motion of molecules
• The phase of matter
• Intermolecular forces between the particles
○ Stronger intermolecular forces mean higher potential energy
○ Weaker intermolecular forces mean lower potential energy
○ Strength of the IMF is liked to the phase of the substance
How is internal energy of a system increased?
by doing work on it and adding heat
How is internal energy of a system decreased?
by losing heat to its surroundings and changing phase from gas to liquid or liquid to solid
What is the lowest temperature on the thermodynamic scale
Absolute zero, temperature at which the molecules in a substance have zero kinetic energy
What happens when the temperature of a substance increases?
- the total energy of the molecules increases
- Since temperature is a measure of average kinetic energy, only an increase in the average kinetic energy of the molecules will result in an increase in temperature of a substance
- Due to thermal expansion, when the temperature of a substance increases, the potential energy of the molecules also increases,
What is the relationship between internal energy and temperature
Temperature and internal energy are directly proportional to each other
ΔU ∝ ΔT
Phase change
another way of saying a state change
What happens during a change in phase?
When a substance reaches a certain temperature, the kinetic energy of the molecules will stop increasing and the energy will go into increasing its electrostatic potential energy
This breaks the bonds between the molecules, causing them to move further apart and this leads to the change in phase.
What happens when a substance is changed form solid to liquid?
the electrostatic potential energy of the molecules increase
The bonds between molecules break and the molecules move further apart
The kinetic energy remains the same, meaning the temperature will remain the same, even though the substance is still being heated.
What happens when a substance is changed from gas to liquid?
• Electrostatic potential energy of molecules decreases
• Bonds form between molecules and the molecules move closer together
• Kinetic energy remains the same, meaning the temperature will remain the same, even if the substance is being cooled
Potential energy in a solid
electrostatic forces between atoms are very large, so electrostatic potential energy has a large negative value
Negative as it requires a lot of energy to break apart bonds
Potential energy in liquids
electrostatic forces between atoms are small but present, so electrostatic potential energy has small negative value
Potential energy in gases
electrostatic force between atoms are negligible, so the electrostatic potential energy is zero
Specific Heat Capacity
Amount of thermal energy required to raise the temperature of 1kg of substance by 1 degree
What is SHC measured in?
Joules per kilogram
When is SHC mainly used?
considering liquids and solids
What happens to thermal energy if the mass of a material increases
Greater mass, more thermal energy required to raise the temperature
What happens to thermal energy if the change of temperature of a material increases
greater change in temperature, higher thermal energy required to achieve change
What does a low SHC mean?
heats and cools quickly
Describe the SHC of conductors
low SHC, make them excellent conductors of heat due to their free electrons
SHC Equation
E = mcΔθ
Where:
E = change in thermal energy (J)
m = mass of the substance (kg)
c = specific heat capacity of the substance (J kg−1K−1 or J kg−1 °C−1)
Δθ = change in temperature (K or °C)
Specific Latent Heat
energy required to change the state of a substance
Is there a temperature change when a substance changes state?
No
What are the two types of latent heat?
- specific latent heat of fusion
- specific latent heat of vaporisation
Specific latent heat of fusion
The thermal energy required to convert 1kg of solid to liquid with no change in temperature.
Specific latent heat of vaporisation
The thermal energy required to convert 1kg of liquid to gas with no change in temperature
Specific latent heat
E = mL
Why is latent heat of vaporisation more than fusion
energy required to increase the molecular separation until they can flow freely over each other
Procedure to determine SLH of fusion
- Place a beaker on each balance
○ Leaving the beaker on the balance, zero the scale - Arrange a funnel, clamped above each beaker
- Set up an immersion heater
○ Connect to the power source
○ Add an ammeter in series and a voltmeter in parallel - Place the immersion heater in one of the funnels
- Measure out 50-100g of ice
○ Add the same mass of ice to each beaker
○ Record this value - Turn on the immersion heater and start the stop watch
- Record the potential difference and current
- After a suitable period of time (around 5-10 minutes) remove the funnels, stop the stop watch and turn off the heater
Record the mass of water in the beaker
Analysis of determining SLH of fusion
- The energy supplied to the ice can be calculated using the equation:
energy = current x potential difference x time - Using the values for current, potential difference and time, calculate the energy supplied
- The specific latent heat of fusion can be calculated using the equation:
energy = mass x specific latent heat - The change mass is equal to the mass of water collected
○ To take into account melting due to heat transfer from the surroundings find the difference in mass between the two beakers of water
○ This gives the change in mass due to the energy supplied by the heater - Calculate the mass of the melted ice and convert it into kg
○ Δm = mA - mB
○ Mass in g ÷ 1000 = Mass in kg
Calculate the specific latent heat of fusion of ice to water using the equation for specific latent heat
Procedure to determine the Specific Latent Heat of Vaporisation
- Connect the double-walled glass vessel to the condenser
○ Place the collecting flask at the end of the condenser - Set up an immersion heater
○ Connect to the power source
○ Add an ammeter in series and a voltmeter in parallel - Place the immersion heater in the fluid
- Turn on the immersion heater and start the stop watch
- Record the potential difference and current
- After a suitable period of time (around 5-10 minutes), stop the stop watch and turn off the heater
Record the mass of water collected in the conical flask
Methods to Determine the Specific Heat Capacity of a Solid
Method for a Solid
1. Assemble the apparatus (not here sadly)
2. Measure the initial temperature of the substance
○ Record the value
3. Turn on the power supply and start the stop-clock
4. Take readings of the voltage and current
○ Record these values
5. After 5 minutes (300 seconds) switch off the power supply, stop the stop-clock
6. Monitor the thermometer
○ Record the highest temperature reached
This may be a few minutes after the power supply is switched off
Methods to Determine the Specific Heat Capacity of a Liquid
- Assemble the apparatus as shown in the diagram above
- Measure the mass of the liquid
○ Record the value - Measure the initial temperature of the substance
○ Record the value - Turn on the power supply and start the stop-clock
- Take readings of the voltage and current
○ Record these values - After 10 minutes (600 seconds) switch off the power supply, stop the stop-clock
- Monitor the thermometer
○ Record the highest temperature reached
This may be a few minutes after the power supply is switched off
